1. Field of Invention
The invention relates to alignment errors in bi-directional printing.
2. Description of Related Art
Image recording devices such as, for example, inkjet printers, fire drops of recording fluid from rows of nozzles of a print head. The nozzles are usually fired sequentially in groups beginning at one end of the head and continuing to the other end of the head. While the nozzles are being fired, the head moves at a rate designed to advance it by a resolution distance before the next firing sequence begins. As an example, for a 300 dot per inch (dpi) resolution with a firing frequency of 5.5 KHz, the print head moves 84.7 .mu.m across the page in 182 .mu.sec. If, for example, the velocity of the drop of recording fluid is 7 m/sec and the distance from the print head to the paper is 1.1 mm, the drop will require 157 .mu.sec to reach the paper. During this time, the drop will drift sideways by 73 .mu.m due to the motion of the print head. If the nozzles are not fired simultaneously, the row of nozzles is usually tilted so that drops fired from all nozzles land in a substantially vertical column.
It is often desirable to operate the print head bi-directionally to enhance productivity. The sideways drift of the print head between the location of the drop of recording fluid being fired and the location of the dot on the paper results in a bi-directional misalignment of two drops, one in each printing direction, fired at the same print head location. In the example given above, where the sideways drift is 73 .mu.m in the direction the printhead is moving, this misalignment would be 146 .mu.m for bi-directional printing. To compensate for this misalignment, the drops fired while the print head is moving in the reverse direction should be fired before the print head reaches the location at which the drop in the forward direction was fired. This firing lead time should be twice as long as the time required for a drop to reach the paper. In the above example, this would be 314 .mu.sec (2.times.157 .mu.sec).
The appropriate amount of lead time is determined by the printer, typically by means of a test pattern where the user is allowed to choose from the best aligned of a series of vertical lines. This procedure works relatively well where all of the nozzles fire drops at the same velocity.